Conventionally, liquid crystal display devices have frequently been used as computer display devices by virtue of their light weight, thin shape, low power consumption, and other advantages, but as screen sizes have grown, there has been a gradual increase in the cases of use as television sets. Because these liquid crystal display devices are hold-type display devices in which display of an image is continued for one frame interval, there is occurrence of motion blurring phenomena (hereafter called “edge blur”), in which the outlines of images are blurred during video display, and so video display performance comparable to that of the CRTs (cathode-ray tubes) generally used in television sets is not obtained.
Numerous research reports have been published concerning the principle of occurrence of the above edge blur (for example, see Taishiro Kurita, “Hold gata Display ni okeru Douga Hyouji no Gashitsu” (Picture Quality of Movie Display in a Hold-type Display), IEICE Tech. Rep., EID99-10, 1999, pp. 55-60), and as a liquid crystal display device in which edge blur is improved, there is a display in which frequency-doubled image signals and black display signals are written periodically within one field interval (see for example Japanese Patent Laid-open No. 2002-31790).
FIG. 8 is a block diagram showing the configuration of a conventional liquid crystal display device. The liquid crystal display device shown in FIG. 8 comprises a signal conversion portion 101, driving pulse generation portion 102, source driver 103, gate driver 104, and liquid crystal panel 105. The signal conversion portion 101 frequency-doubles the input image signal for each line, using an input synchronization signal as reference, converts the image signal into a frequency-doubled signal consisting of a frequency-doubled image signal and frequency-doubled black display signal, and outputs the frequency-doubled signal to the source driver 103. The driving pulse generation portion 102 outputs a source driver control signal and gate driver control signal, with the input synchronization signal as reference, to control the source driver 103 and gate driver 104. The source driver 103 applies the voltage to be applied to individual pixels within the liquid crystal panel 105 to the source lines SL1 to SL10. The gate driver 104 applies, to the gate lines GL1 to GL10, a voltage to set each of the pixels in the liquid crystal panel 105 to the on state or the off state. At this time, each of the gate lines GL1 to GL10 of the liquid crystal panel 105 is selected twice within one field interval, and an image signal and black display signal are written once each to the pixels on each of the gate lines GL1 to GL10. Hence black insertion driving, in which a black display signal is written periodically while writing image signals, can be realized.
FIG. 9 is a drawing showing the change with time in pixel brightness in the conventional liquid crystal display device of FIG. 8. As shown in FIG. 9, one field interval consists of an image display interval T1 and a black display interval T2, and a given pixel is driven as illustrated so that black is periodically displayed. In this case, the driving method in the liquid crystal display device is pseudo-impulse driving (hereafter called “pseudo-impulse driving”), and the edge blur which occurs in video display can be improved.
However, although edge blur can be improved through pseudo-impulse driving in the above-described liquid crystal display device, each field interval comprises a black display interval, so that the average brightness is decreased. The longer the black display interval is made in order to improve edge blur, the more pronounced is this tendency, so that brightness is decreased and satisfactory video display is not possible.
Further, because the amount of motion of an image in the display of video changes variously according to the type of video, the required black display interval length also changes variously. Hence when a black display interval appropriate to an image having an average motion amount is set, edge blur can be reduced sufficiently for images with a small amount of motion, but the brightness is reduced unnecessarily, and satisfactory video display is not possible.
Further, when using a TN (Twisted Nematic) mode in the above-described liquid crystal display device, the driving response time is slow at approximately 16 ms, so that even if a black display interval is set, edge blur may remain due to the slowness of the driving response of the liquid crystal panel.